Preform, i.e. a solid piece of solder, in this case a thin sheet with an
area slightly smaller than the ground slug. We have them and Indium has
them, and likely the other solder suppliers have them too. This will only
work if the ground contact on the QFN wets well and the problem is
insufficient solder. Some preforms are fluxed, and that may help the
wetting a little.
If the ground contact on the QFN wets, then a (preform, n.) may (perform,
v.) well.
You could also try capping the vias, already mentioned, or underprinting
the perimeter pads a little to compensate for the solder lost to the vias.
Karen Tellefsen - Electrical Testing
Alpha / 109 Corporate Blvd./ S. Plainfield, NJ 07080
[log in to unmask]
908-791-3069
From: Joyce Koo <[log in to unmask]>
To: <[log in to unmask]>,
Date: 02/22/2013 12:43 PM
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.
Sent by: TechNet <[log in to unmask]>
Perform time... ;-)
Joyce Koo
Materials Researcher - Materials Interconnect Lab
Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kuczynski, Michael
Sent: Friday, February 22, 2013 12:37 PM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.
As a librarian and fellow PCB guy her's my 2cents..
I have a feeling the vias are robbing the solder.
Are you using the vias because the slug tied to GND? Or doing it for
thermal reasons? Or both?
If neither, take them out next spin of the PCB.
If you need them, reduce the finished drill size to .008 or smaller, as
well as reducing the number of vias under there to 4 or 5.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Alan Young
Sent: Friday, February 22, 2013 12:23 PM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.
Hi folks,
Please forgive me if this topic does not belong to this group.
Let me explain my predicament.
I'm a PCB layout guy who has been asked to look at a soldering issue we are
having with a 48-pin QFN 7mmx7mm [EMBER CORP - EM357-RTR], the solder slug
under the part is not soldering/bonding to the PCB. The pcb footprint is
made per the manufactures specification with a 5.3mm x 5.3mm solder slug
pad with 9 vias finished drill size of 0.254mm. There are no soldermask
features on the component or solder side over the vias.
When the board has been reflowed, all of the 48 pins solder well but the
slug has not bonded to the pcb. There are signs of solder in the slug vias
on the opposite side of the board. If the part is ripped off the board, the
slug is clean and solder free.
I do not have any information about the paste screen setup or the oven
profile.
The questions I have are;
1 - Any ideas on why this is happening?
2 - Are the vias in the slug causing this problem?
3 - Would soldermaks capping the vias on the component side help resolve
this?
4 - Would plugging (no capping) of the vias with epoxy help?
5 - Do north African sparrows fly faster then European sparrows (just
seeing if anybody is reading this :-)
I do have some photos and x-rays of the device if this will help.
Again any help or comments would be greatly appreciated.
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